1. Field of the Invention
The present invention relates generally to optical communication systems, and particularly to detecting or monitoring optical impairments, such as polarization mode dispersion (PMD) which includes chromatic, polarization dependent, and other optical losses in such systems.
2. Technical Background
With the ever increasing bit-rates or data rates in fiber-based optical communication systems, signal degradation from distortion and fading due to well-known but unpredictable optical impairments, such as polarization mode dispersion (PMD) has become a pressing concern. PMD originates from the unwanted and residual birefringence in optical fibers. The birefringence causes light pulses to split into polarization modes that propagate with different velocities along a fiber link. With a simplistic view of the differential group delay (DGD), the differential time of flight between two specific polarization components over a length of fiber, causes the broadening of optical pulses, thus signal degradation.
As is known, an optical pulse exiting a fiber can be decomposed into two components in two mutually orthogonal polarization states that can be arbitrarily chosen. In the presence of PMD, it is convenient to decompose the pulse, at an unpredictable state of polarization (SOP) into two specific polarization components, i.e. the principal states of polarization (PSP). Although there is a differential group delay (DGD) between the two components, in the first order of approximation, each constituent pulse (in a PSP) possesses the same shape as the original pulse (input). Systems, including a polarization transformer and a differential group delay device, designed for the first order PMD compensation separates the two PSPs and inserts an opposite relative group delay so that the overall pulse recovers its original shape.
As the PMD of a fiber varies with time in both magnitude and orientation, an ideal method of compensation is real-time or active so that data transmission is uninterrupted while the PMD is continuously monitored and compensated. A crucial technology in realizing active PMD compensation is to monitor and determine the PSPs and the DGD in a fiber link in real time for a feedback system to automatically compensate for the signal distortion caused by the PMD.
Various schemes have been demonstrated for active compensation or mitigation of PMD at the receiving end. In one system, the lightwave pulses are converted to electrical pulses whose spectrum is analyzed. Schemes can be derived to extract certain frequency components as the distortion indicator that drives the adaptive or compensating components. The conventional feedback system samples the optical signal with a tapped feedback control loop that includes a high-speed photodetector. Similar methods of sensing pulse distortion with high-speed receivers are used in other PMD compensators. Common to these techniques, one or more high-speed photodetectors must be used in the tap. In order to obtain a workable signal-to-noise ratio for extracting certain frequency components from the spectrum as the distortion indicator, substantial optical power must be extracted from the fiber and expensive electronics of microwave frequencies must be included. If the signal-to-noise ratio is not high enough, signal ambiguity may cause control difficulties.
Therefore, there is a need to create a simple, easy to implement feedback system in which the adaptive components are driven by an electrical signal extracted from the optical pulses for indicating optical impairments without using high-speed detectors and high-speed electronics, while obtaining a clear and unambiguous control signal to enable a practical algorithm for fast and automatic PMD compensation.